Set GERRIT_REFSPEC defaults for Gerrit Trigger jobs

According to Gerrit Trigger documentation if you set a default value for
GERRIT_REFSPECT you can trigger jobs using the build now button rather
than it always failing.

Might as well set this so that it's easier to test these job types on
the sandbox.

See: https://wiki.jenkins-ci.org/display/JENKINS/Gerrit+Trigger

Change-Id: I7cc03920016de09d35fb6f881067c9cb401522fc
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>